Operation of lean burn engines, e.g., diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy, and have very low emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Diesel engines, in particular, also offer significant advantages over gasoline engines in terms of their durability, and their ability to generate high torque at low speed. However, exhaust from lean burn gasoline engines is characterized by relatively high emissions of NOx as compared to conventional gasoline engines that operate at or close to stoichiometric air/fuel conditions. Effective abatement of NOx from lean burn engines is difficult to achieve because high NOx conversion rates typically require reductant-rich conditions. Conversion of the NOx component of exhaust streams to innocuous components generally requires specialized NOx abatement strategies for operation under fuel lean conditions.
Efficient reduction of nitrogen oxides (NOx═NO+NO2) from diesel and lean-burn gasoline exhaust is important to meet future emission standards and improve vehicle fuel economy. Reduction of NOx emissions from an exhaust feedstream containing excess oxygen to meet various regulatory requirements is a challenge for vehicle manufacturers. For example, it is estimated that compliance with Bin 5 regulations in the United States may require an aftertreatment system capable of 70-90% NOx conversion efficiency on the FTP (Federal Test Procedure) cycle based on currently anticipated engine-out NOx levels. One such strategy for the abatement of NOx in the exhaust stream from lean burn engines uses NOx storage reduction (NSR) catalysts, which are also known in the art as “NOx traps.” NSR catalysts contain NOx sorbent materials capable of adsorbing or “trapping” oxides of nitrogen under lean conditions and platinum group metal components to provide the catalyst with oxidation and reduction functions. In operation, the NSR catalyst promotes a series of elementary steps which are depicted below in Equations 1-5. In an oxidizing environment, NO is oxidized to NO2 (Equation 1), which is an important step for NOx storage. At low temperatures, this reaction is typically catalyzed by the platinum group metal component, e.g., a platinum component. The oxidation process does not stop here. Further oxidation of NO2 to nitrate, with incorporation of an atomic oxygen, is also a catalyzed reaction (Equation 2). There is little nitrate formation in absence of the platinum group metal component even when NO2 is used as the NOx source. The platinum group metal component has the dual functions of oxidation and reduction. For its reduction role, the platinum group metal component first catalyzes the release of NOx upon introduction of a reductant, e.g., CO (carbon monoxide) or HC (hydrocarbon) (Equation 3) to the exhaust. This step may recover some NOx storage sites but does not contribute to any reduction of NOx species. The released NOx is then further reduced to gaseous N2 in a rich environment (Equations 4 and 5). NOx release can be induced by fuel injection even in a net oxidizing environment. However, the efficient reduction of released NOx by CO requires rich conditions. A temperature surge can also trigger NOx release because metal nitrate is less stable at higher temperatures. NOx trap catalysis is a cyclic operation. Metal compounds are believed to undergo a carbonate/nitrate conversion, as a dominant path, during lean/rich operations.
Oxidation of NO to NO2 NO+1/2O2→NO2  (1)
NOx Storage as Nitrate2NO2+MCO3+1/2O2→M(NO3)2+CO2  (2)
NOx ReleaseM(NO3)2+2CO→MCO3+NO2+NO+CO2  (3)
NOx Reduction to N2 NO2+CO→NO+CO2  (4)2NO+2CO→N2+2CO2  (5)
In Equations 2 and 3, M represents a divalent metal cation. M can also be a monovalent or trivalent metal compound in which case the equations need to be rebalanced.
While the reduction of NO and NO2 to N2 occurs in the presence of the NSR catalyst during the rich period, it has been observed that ammonia (NH3) can also form as a by-product of a rich pulse regeneration of the NSR catalyst. For example, the reduction of NO with CO and H2O is shown below in equation (6).
Reduction of NO to NH3 2NO+5CO+3H2O→2NH3+5CO2  (6)
This property of the NSR catalyst mandates that NH3, which is itself a noxious component, must also now be converted to an innocuous species before the exhaust is vented to the atmosphere.
An alternative strategy for the abatement of NOx under development of mobile applications (including treating exhaust from lean burn engines) uses selective catalytic reduction (SCR) catalyst technology. The strategy has been proven effective as applied to stationary sources, e.g., treatment of flue gases. In this strategy, NOx is reduced with a reductant, e.g., NH3, to nitrogen (N2) over an SCR catalyst that is typically composed of base metals. This technology is capable of NOx reduction greater than 90%, thus it represents one of the best approaches for achieving aggressive NOx reduction goals.
Ammonia is one of the most effective reductants for NOx at lean condition using SCR technologies. One of the approaches being investigated for abating NOx in diesel engines (mostly heavy duty diesel vehicles) utilizes urea as a reductant. Urea, which upon hydrolysis produces ammonia, is injected into the exhaust in front of an SCR catalyst in the temperature range 200-600° C. One of the major disadvantages for this technology is the need for an extra large reservoir to house the urea on board the vehicle. Another significant concern is the commitment of operators of these vehicles to replenish the reservoirs with urea as needed, and the requirement of an infrastructure for supplying urea to the operators. Therefore, less burdensome and alternative sources for supplying the reductant NH3 for the SCR treatment of exhaust gases are desirable.
Emissions treatment systems that utilize the catalytic reduction of NOx in the exhaust to generate NH3, in place of an external reservoir of NH3 or NH3 precursor are known in the art. In other words, a portion of the NOx component of the exhaust is used as an NH3 precursor in such systems. For instance, U.S. Pat. No. 6,176,079 discloses a method for treating an exhaust gas from a combustion system that is operated alternately in lean and rich conditions. In the method, nitrogen oxides are intermediately stored during lean operation, and released during rich operation to form NH3 that is stored. The stored NH3 can be released, and thereby reduce nitrogen oxides during a subsequent lean operation.
Selective catalytic reduction of NOx using hydrocarbons (HC—SCR) has been studied extensively as a potential alternative method for the removal of NOx under oxygen-rich conditions. Ion-exchanged base metal zeolite catalysts (e.g., Cu-ZSM5) have typically not been sufficiently active under typical vehicle operating conditions, and are susceptible to degradation by sulfur dioxide and water exposure. Catalysts employing platinum-group metals (e.g., Pt/Al2O3) operate effectively over a narrow temperature window and are highly selective towards N2O production.
Catalytic devices using alumina-supported silver (Ag/Al2O3) have received attention because of their ability to selectively reduce NOx under lean exhaust conditions with a wide variety of hydrocarbon species. In addition, diesel fuel could be supplied as a reductant. Diesel fuel does not require additional tanks for diesel-powered vehicles. The diesel fuel can be to the emissions system by changing engine management or by supplying an additional injector of diesel fuel to the emission train.
Despite these various alternatives, there is no practical hydrocarbon SCR catalyst. Therefore, there is a need for an effective method and apparatus to selectively reduce NOx in an exhaust gas stream for vehicles and other applications of lean-burn internal combustion engines.